Television signal control devices and circuitry designed for use in distributing television signals to viewer locations through cable systems are often required to have the capability for selective disablement by the distributor or viewer of a particular television channel without affecting the transmission of other channels throughout the system. Such disabling or jamming circuits, for example, give the viewer the option to purchase and the distributor the option to sell premium television programming. The disabling circuitry must, for economic reasons, be located in the proximity of the subscriber's home but may be activated or de-activated either by service call or, where addressable taps are used, by remote control.
The standard NTSC television broadcast channel has an allocated bandwidth of 6 MHz. The video carrier frequency is 1.25 MHz above the lower channel edge and is amplitude modulated to convey picture and synchronizing information in an approximate 4 MHz range above the carrier frequency. Chroma information is generally centered about a sub-carrier frequency located 3.58 MHz above the video carrier frequency. The audio signal is transmitted by frequency modulation of the sound carrier frequency, which is located 4.5 MHz above the video carrier frequency, leaving 0.25 MHz between the sound carrier frequency and the upper channel edge.
Selective channel control requires that one or more of the carrier frequencies of the signal be perturbed or filtered in some manner. Methods previously used for channel control have included video carrier traps, modulated video carrier traps and continuous wave frequency jamming.
Video carrier traps are generally very narrow, high-Q, notch filters tuned to the video carrier frequency and having a notch depth with order of 50 Db of attenuation. When switched into the signal path feeding the subscriber location, the trap causes an attenuated signal which results in presentation of a noisy and distorted picture on the jammed channel. Unfortunately, the carrier trap often adversely affects the sound and the picture on the lower adjacent channel and often does not remove the audio on the channel to be controlled. Therefore, carrier trap systems are generally limited to use with the lower frequency VHF channels or to use with channels in which a lower adjacent audio signal does not exist.
Modulated carrier traps are simply notch filters which are similar to those described in the preceding paragraph but which are switched into and out of the subscriber signal path at typical frequency rates of 30 Hz, 60 Hz or 15,750 Hz, resulting in square-wave modulation of the received video signal. Because television receivers are tuned to operate with vertical and horizontal sweep synchronizing information having frequencies of 60 Hz and 15,750 Hz, the modulated trap disrupts operation of the synchronizing and sweep circuits resulting in picture distortion. However, the switching of the trap usually produces a small but visible amount of interfering modulation on the lower and perhaps the upper adjacent channels. Furthermore, if the trap is made narrow enough to avoid adjacent channel interference, the sound portion of the controlled channel may continue to be transmitted.
Continuous wave frequency jamming may be accomplished by directional coupling of a video carrier frequency into the signal path directed to the subscriber location. The continuous wave frequency may be amplitude modulated at sweep frequency rates to further disrupt the viewed picture. However, neither the continuous wave jamming frequency nor the modulated continuous wave jamming frequency will remove the audio content from the signal transmitted to the subscriber location.
Another method which has been used to secure a premium channel is the insertion at the system transmitter location of a signal having a frequency located between the video and chroma carrier frequencies, the signal being modulated to produce a relatively narrow-band interference spectrum which produces both video and audio distortion. The jamming signal is removed for paying subscribers by introducing an equivalently narrow-band notch filter at the input to the television set. Unfortunately, this system offers limited security against unauthorized access since a non-paying subscriber may surreptitiously filter out the jamming signal, thus defeating its intended purpose.